1. Field of the Invention
This invention relates to a circuit board, especially to a circuit board used for a power semiconductor module, in which a metal circuit plate which acts as a conductive circuit plate is bonded to one face of an insulating-ceramic substrate, and a metal heat sink is bonded to another face. Especially this art can be applied effectively to a power semiconductor module for which high reliability against thermal cycles is demanded.
2. Description of the Related Art
In recent years, power semiconductor modules (IGBTs, power MOSFETs, etc.) which work at high voltage, or at high current, are used as an inverter for electric vehicles. As a substrate used for the power semiconductor module, a circuit board in which a conductive metal plate (metal circuit plate) is bonded to one face (upper face) as a circuit, and a metal plate for heat dissipation (metal heat sink) to another face (lower face), of an insulating-ceramic substrate which is composed of aluminum nitride or silicon nitride ceramics, is used widely. A copper plate or an aluminum plate is used as these metal plates and semiconductor devices, etc., are mounted on the upper face of the metal circuit plate. An active metal brazing method in which a metal brazing material is used, or a direct bonding copper method namely, by which a copper plate is bonded directly, is used for bonding the metal plates to the insulating ceramics.
However, in the power semiconductor module using the circuit board in which the metal circuit plate and the metal heat sink are bonded to the insulating-ceramic substrate, thickness of the metal circuit board and of the metal heat sink may be made comparatively thick at 0.5 mm or larger, so that high current can be sent. In this case, when copper with high thermal conductivity is used especially for both the metal circuit plate and the metal heat sink, thermal expansion coefficients of these are different from the insulating ceramics greatly (for example, 3.0×10−6/K for silicon nitride ceramics, and 16.7×10−6/K for copper). Therefore, high thermal stress occurs in cooling process after the bonding, or by thermal cycles during operation of the power semiconductor module. This stress exists as compressive or tensile residual stress near the bonded part of the insulating-ceramic substrate. This residual stress may make the insulating-ceramic substrate cracked, or may be a cause of poor electrical isolation, or delamination of the metal circuit plate or the metal heat sink.
Actually, for such a circuit board, a reliability with which efficient thermal radiation is kept against predetermined thermal cycles from −55° C. to 150° C., is needed. Endurance against 200 thermal cycles or more, 1000 thermal cycles or more, or also 3000 thermal cycles or more, is needed depending on the applied field. When especially used for a hybrid car, an electric vehicle, a train, an airplane, etc., high-reliability against the thermal cycle is needed.
From this point of view, although an aluminum nitride substrate has high thermal conductivity, since mechanical strength and fracture toughness are low and it is not reliable, it is difficult to use it as the insulating ceramic substrate. Since silicon nitride substrate has thermal conductivity comparatively high as the insulating ceramic substrate, and has also high mechanical property, it is thought that a reliable circuit board can be made using it.
However, even when the silicon nitride substrate with high mechanical property is used as the insulating-ceramic substrate, a crack may have been generated by the thermal stress during the heat cycles.
Mostly, this crack may have been generated in the peripheral parts of the pattern of the metal circuit plate, especially in the corners, and this crack may have degraded the isolation voltage and mechanical strength of the silicon nitride substrate, and when voltage was applied to the mounted semiconductor chip, the silicon nitride substrate may have been broken down electrically. Therefore, the reliability of the semiconductor module in which semiconductor chip is mounted on such a circuit board, was not enough.
As an art in the circuit board which is aimed to improve the reliability against the thermal cycles, the art making the fracture toughness value of the silicon nitride substrate higher than 6.5 MPa·m1/2, is indicated by patent documents 1 (JP 2002-201075 A). Here, while setting three point bending flexural strength of the used silicon nitride substrate at 500 MPa or higher, and also setting the fracture toughness value of that at 6.5 MPa·m1/2 or higher, generation of the crack by the thermal stress is suppressed. Here, the fracture toughness value shows the endurance against the generation of the crack after indent is given on the insulating-ceramic substrate. This value is calculated from dimensions of the indentation and of the cracks generated by the indentation after Vickers indenter is pressed to the point of measurement, by IF method described in JISR1607 (Japan). Namely, in this case, by raising the fracture toughness value of the insulating-ceramic substrate material itself, it cannot be broken easily.
Also in patent document 2 (JP 2005-26252 A), it is indicated that the fracture toughness value of the insulating-ceramic substrate is made higher by bonding a metal circuit plate etc., to the insulating-ceramic substrate. Here, it was shown that the fracture toughness value especially on the face (a face to which the metal circuit plate is bonded) of the insulating-ceramic substrate is increased, by bonding a metal circuit plate, and the endurance against the thermal cycles is improved. In this case, by making the fracture toughness value on the face of the insulating-ceramic substrate high, it becomes hard for a crack to expand forward from a face of the insulating-ceramic substrate, and the substrate becomes hard to be broken.    Patent documents 1: JP 2002-201075 A    Patent documents 2: JP 2005-26252 A
However, semiconductor modules being operated with high electric power are used recently. In this case, corresponding to operation of the semiconductor chip with high electric power, a higher thermal radiation characteristic will be required more for the circuit board, therefore, thicker metal circuit plate or thicker heat sink is used. Therefore, the influence of the thermal expansion difference between the insulating-ceramic substrate and the metal circuit plate or the metal heat sink becomes still greater, and the camber of the substrate during thermal cycles is still larger. In such a case, it was difficult to keep enough endurance of the circuit board only by the aforementioned method.